The human fetus and neonate are unduly susceptible to severe infection by intracellular pathogens, including cytomegalovirus (CMV), herpes simplex virus (HSV), Toxoplasma gondii, and Listeria monocytogenes. Macrophages are a critical component of host defense against such intracellular pathogens. Macrophages, which are fetal in origin, will be isolated from human placentas and their antimicrobial activity against these organisms will be assessed; macrophages will be obtained by pressing minced placenta fragments through fine-meshed sieves and the suspensions obtained will be purified by sequential density gradient centrifugation, velocity sedimentation at unit gravity, and differential adherence to glass or plastic. The ability of these macrophages to phagocytose or absorb CMV and HSV and to kill or inhibit intracellular replication of these viruses will be assessed quantitatively and on a single cell basis; their ability to phagocytose and kill or inhibit intracellular replication of T. gondii and L. monocytogenes will also be determined. In addition, the activity of placental macrophages--which have been incubated with culture supernatants from mitogenically-stimulated neonatal and adult lymphocytes--against these organisms will be studied in parallel with macrophages not so treated to assess whether these cells can be "activated" to a state of enhanced antimicrobial activity. Further, because infection with CMV and HSV alters host resistance to other infections, the activity of placental macrophages--which have been infected with CMV or HSV in vitro--against T. gondii and L. monocytogenes will be studied in parallel with uninfected macrophages. The role of oxygen-dependent and independent mechanisms in the antimicrobial activity of normal, activated, and virus-infected macrophages will be investigated. These studies will increase our understanding of human fetal and neonatal defense mechanisms against infection and suggest means to enhance these defenses.